Explosive growth in electronics technology has resulted in electronic devices used all around us in seemingly every facet of life. For example, communications equipment, toys, computers, automobiles, personal digital assistants (PDAs), household appliances, medical equipment, etc., all include increasingly powerful electronic circuits. As electronic devices become more powerful, however, their design and manufacture has become more complex and sensitive, particularly as their speed increases.
Although the design and manufacture of electronic circuits may be carried out in a number of ways, two steps in the design process are practically universal: first, the logical or functional design of the circuits, and second, the physical design of the circuits. In the first step, a circuit design is created in which circuit elements are selected and interconnected to implement the desired functionality of the circuit. The result of this functional design step is a logical circuit design file describing the interconnections in the circuit, such as “L1_pin A should connect to L2_pin B”.
The second of these two design steps is to generate a physical circuit layout from the logical circuit design for the desired product, such as an integrated circuit (IC), an IC package, a printed circuit board, etc. The circuit layout can be used to form a mask which can be provided to a foundry for fabrication. For example, the circuit layout describes the conductive lines or traces including their width, shape and position, and the conductive vias which connect the traces on different circuit layers.
Many electronic design automation (EDA) software packages are available to aid in these two steps of electronic circuit design, including place-and-route tools and package design tools such as Allegro and Advanced Package Designer (APD), available from Cadence Design Systems, Inc. of San Jose, Calif. Allegro enables a designer to place (assign locations to circuit elements) and route (connect circuit elements with traces) a printed circuit board based on a logical circuit design and constraints specified by the designer. Similarly, APD is a software application that enables a package designer to design IC packages, laying out components and connections based on constraints or design rules specified by the designer. Many other EDA software packages are also available from other companies.
The design constraints specified by the designer may be used to indirectly control some physical and electrical characteristics of the product being designed. For example, the constraints may include available trace widths, minimum trace spacing, minimum and maximum trace length, etc., which impact physical and electrical characteristics such as signal delay and distortion. However, characteristic impedance values for traces and vias are not always directly controlled and may vary from the desired value. Improper characteristic impedance values may cause errors due to signal distortion such as reflection or ringing, particularly in high-speed circuits that are sensitive to signal distortion.